A communication system is a facility which enables communication between two or more entities such as user terminal equipment and/or network entities and other nodes associated with a communication system. The communication may comprise, for example, communication of voice, electronic mail (email), text messages, data, multimedia and so on.
The communication may be provided by a fixed line and/or wireless communication interface. A feature of wireless communication systems is that they provide mobility for the users thereof. An example of communication systems providing wireless communication are public land mobile networks (PLMN). An example of the fixed line system is a public switched telephone network (PSTN). A communication system can also comprise a plurality of wireless interfaces and access technologies, such that a terminal can be configured to be capable of communicating with the communication system using one or several of said plurality of access technologies at a given time. Examples of such access technologies and corresponding wireless interfaces include, but are not limited to, WLAN, WCDMA, CDMA2000, EDGE, Bluetooth, HiperLAN, WIMAX and digital satellite communications.
A communication system typically operates in accordance with given standards or specifications which set out what the various elements of a system are permitted to do and how that should be achieved. For example, the standards or specifications may define if the user or more precisely the user equipment is provided with a circuit switched service or a packet switched service or both.
Communication protocols and/or parameters which should be used for the connection are also typically defined. For example, the manner in which communication should be implemented between the user equipment (UE) and the elements of the communication network is typically based on predefined communication protocols. In other words, a specific set of “rules” on which the communication can be based needs to be defined to enable the user equipment to communicate via the communication system.
So called third generation communication systems are being introduced. These so called third generation systems typically use Code Division Multiple Access (CDMA) techniques such as Wideband Code Division Multiple Access (WCDMA).
In certain third generation systems base stations are referred to as Node Bs. Node Bs (or base stations) are arranged to communicate via a wireless interface with user equipment. The Node Bs are connected to Radio Network Controllers (RNCs). RNCs and Node Bs make a UMTS Terrestrial Radio Access Network (UTRAN).
The node Bs are responsible for scheduling the radio transmissions of certain channels such as the High Speed Downlink Shared Channel (HSDSCH) and Enhanced Uplink Dedicated Channels (EDCH).
Soft Handover (SHO) refers to a feature used by the CDMA standard, where a user equipment is simultaneously connected to two or more Node Bs during a communications session. This technique is a form of mobile-assisted handover for CDMA user equipment. The user equipment makes various signal quality/power measurements of a neighbouring Node Bs, and determines whether or not to add or remove the Node Bs from an active set to which the user equipment is connected. This is known as soft handover addition/deletion. The active set is a list of Node Bs held in the user equipment indicating which Node Bs the user equipment has an active radio link with. The user equipment can send signals to, and receive signals from, the Node Bs in the active set during a communication session.
Due to the properties of the CDMA signalling scheme, it is possible for a CDMA user equipment to simultaneously receive signals from two or more nodes that are transmitting the same bit stream on the same channel. If the signal power from two or more nodes is nearly the same, the user equipment can combine the received signals in such a way that the bit stream is decoded much more reliably than if only one base station were transmitting to the user equipment. If any one of these signals fades significantly, there will be a relatively high probability of having adequate signal strength from one of the other nodes.
On the “reverse” link or uplink (user equipment to Node B), all the Node Bs that are actively supporting a communications session in soft handover send the bit stream that they receive back to the RNCs, along with information about the quality of the received bits. The RNC examines the quality of all these bit streams and dynamically chooses the bit stream with the highest quality. Again, if the signal from one Node-B degrades rapidly, the chances are still good that a strong signal will be available at one of the other Node Bs that is supporting the call in soft handover.
The transmissions sent between the user equipment and the plurality of Node Bs to which it is connected in soft handover must be synchronized (soft handover synchronization). A Node B is synchronized when it is added to the active set.
Methods of synchronization are known in the art. However, a problem with known arrangements is that synchronization is difficult for non-continuous transmissions.
In 3rd Generation Partnership Project (3GPP) Technical Document R1-051448 published by Ericsson on 1 Nov. 2005 (which is incorporated herein by reference), opportunities for resource saving in the Node B receiver are discussed. It is stated that in the concept UL DPCCH Gating (Uplink Dedicated Physical Control Channel Gating), the start positions for the transmissions during packet traffic activity can be restricted to certain subframes/frames, for example to the subframes/frames given by the periodic DPCCH transmission pattern. The allowed start positions must of course be rather frequent in order not to introduce unacceptable delays. This would decrease the receiver resource needs significantly. It would also eliminate the need to continuously detect the presence of DPCCH as the receiver would know in advance when the transmission will take place. The document states further that when a radio link is added to the active set during soft handover addition (sync procedure B), it would be beneficial to transmit the DPCCH continuously until the new radio link has been successfully added. If the DPCCH signal is discontinuous, it is harder for the Node B to obtain uplink synchronisation and hence there is a risk that Node B needs to spend more time and/or more resources before it can declare that soft handover synchronization has been achieved. Notably the same solution would be beneficial in all cases when a Node B is establishing the uplink synchronisation, soft handover being just one example of this.
Thus, it has been recognized that discontinuous DPCCH transmission can save resources in the Node B. However, the provision of discontinuous DPCCH makes it difficult to perform the uplink synchronisation in the Node B to which the new radio link is being added. This need occurs for soft handover synchronization as well as during the initial radio link synchronisation when the radio link is being set up for the first time or the UE is performing a hard handover, i.e. a handover in which the old radio links are deleted and (a) new one(s) added. The new Node B synchronizing to the transmission of the uplink signal faces the same uplink synchronisation acquisition problem regardless of whether a hard handover, a new soft handover link or an initial connection is being performed. One solution to this problem would be to provide continuous DPCCH transmission always. However, this would result in a heavy signalling burden on the Node B and render the proposed enhancement of gating the uplink DPCCH useless.
The present invention aims to solve the aforementioned problems.